The price of steem changes daily, and so does the value of posts and upvotes due to that. With a few cent drop in steem or sbd price, you will see a 3 or 4 cent post go to 1 or 0 cents.. For example I had a post that was over $50 a while back, and when the day 7 came around it was at $42. And alot of my less then 10 cent posts or upvotes go down to like 2 or 3 cents sometimes, steem is volitile and it changes so much from hour to hour even sometimes.